20 THE VOYAGE OF H.M.S. CHALLENGER. 



Hook-sacs bearing about twenty long books, placed in tbe bottom of the sac, and 

 several shorter on the lateral walls, of which the extremities come to the same height as 

 those of the bottom hooks when the sac is evaginated. 



The other characters are the same as for the genus. The specimens of this species 

 always show a constriction in the posterior half of the body, behind the dorsal spot, 

 and at the height of the lateral gill. 



Length. — Maximum, 22 mm. 



Colour. — Grey-brown; a thin longitudinal white line, in the middle of the back, 

 extending backwards to the above constriction. 



Challenger Specimens. — Station 158, March 7, 1874 ; Termination Land to Melbourne; 

 lat. 50° l'S.,long. 123° 4' E. 



Station 314a, January 22, 1876 ; Sandy Point to Falkland Islands; lat. 51° 24' S., 

 long. 61° 46' W. 



Between Stations 332 and 333, March 11, 1876; Rio de la Plata to Tristan da 

 Cunha; lat. 36° 22' S., long. 26° 1' W. 



Habitat. — Antarctic Seas, within the isothermal line of 50° F. for August (it often 

 does not even pass beyond the isotherm of 40°); from long. 60° W. to long. 123° E., 

 and probably all around the Antarctic Pole. The extreme observed latitudes are 

 lat. 36° 22' S. (Challenger Expedition, March 11, 1876), and lat. 54° 30' S. 

 (d'Orbigny). The localities, " New Britannia, Marquesas," in the Hamburg Museum, 

 are certainly erroneous. 



Observations. — It is evident from one of the original drawings of Pteropoda by Dr. 

 Hooker (sent to me with the Challenger Pteropoda) that the English Antarctic Expeditin 

 (1840), had obtained Spongiobranchsea australis. But this drawing does not bear any 

 indication of locality. D'Orbigny only knew a single very small specimen of this species 

 (7 to 8 mm. long) ; and it is thus easy to understand why he only saw six suckers on 

 each buccal appendage. He says nothing about the lateral gill ; but he ought to have 

 recognised it, as it may be seen in his figure. 1 It is extraordinary that he says : anus 

 " a, gauche," 2 the anus being on the right side as in all the Gymnosomata. 



I have already 3 expressed the opinion that Trichocyclus dumerili, Esch., 4 is the 

 larva of Spongiobranchsea. I support this interpretation on the length of the labial 

 tentacles of this larva. 5 Johannes Midler had already said 6 that, for this reason, 

 Trichocyclus appeared to him to be more the larva of Clione than of Pneumonoderma ; 



1 Voyage dans l'Amerique meridionale, t. v., pi. ix. fig. 2. 



2 Loc. cit., p. 1.31. 



3 Description d'un nouveau genre de Pteropode Gymnosome, Bull. Sci. Depart. Nord, 1886, p. 218. 



* Bericht iiber die zoologische Ausbeute wahrend der Keise von Cronstadt bis St. Peter nnd Paul ; Oken, Isis, 1825, 

 p. 735, pi. v. rig. 4. 



6 I niay here notice that Esehseholtz's figure is not exact, because it shows the first ciliated ring anterior to 

 these tentacles. 



6 Uebet verschiedene Formen von Seethieren, Archivf. Anat. u. Physiol, 1854, p. 70. 



